I confirm it from my side. As long as you have the key, you can download any untouched Windows identical to yours, burn it on a DVD, then use the key with it. I did it on my HP Pavilion laptop with Windows 7 Home Premium. Then I went to HP's website for the drivers and found them all using the same model code. Easy.
HP can take their recovery system else where
If it is custom built, you only need to know the devices' names and brands and get their drivers from their websites. But before installing any drivers, make sure you install the motherboard's chipset's drivers. Very important to avoid possible compatibilities issues specially with video cards.
Don't worry about it, its easy.
Windows XP has many corporate key's scattered around so it is easy to get original copies. Not sure about its legality tho.